CD-ROM is an extension of CD audio, the high-quality disk technology of the music industry. A conventional CD-ROM disk, measuring only 4-3/4 inches in diameter is capable of storing an equivalent of up to 250,000 pages of text, 1,500 floppy disks, 74 minutes of audio, or thousands of images, any of which can be retrieved in seconds by a computer-based CD-ROM player. CD-ROM products are available to users in a wide variety of fields such as library science, education, publishing, online database services, government, banks, insurance, law, engineering, and medicine.
CD-ROM disks typically have a single, spiraling track that is about three miles long. The track is typically read from the inside out. Data on the track is divided into sectors, each equal in length, containing equal amounts of information and having absolute addresses. FIG. 1 illustrates a standard data format. In this format, each sector 13 on CD-ROM disk 11 comprises 2,352 bytes of information with 12 bytes of synchronization, 4 bytes of identification, 288 bytes of error-correction code and 2,048 bytes of data.
A CD-ROM master is made by use of a high-power laser to form a series of tiny indentions in the surface of a blank forming the single spiral track. The pattern and length of the indentions along the track represent the recorded information digitally. The original blank is used to make a master, typically of wear resistant material, for use in molding replicas, which become the familiar CD-ROM disks. The pattern from the master disk is reproduced on the surface of a polymer substrate, which is then a copy of the original.
The stamped replicas are coated with a highly reflective material, typically vapor deposited aluminum, and a protective coating is applied over the thin reflective film.
Early CD-ROM disks comprised digitized analog information designed to be used primarily in linear format with limited user control of playback. Other CD-ROM disks were programmed to use the disk player's microprocessor and limited internal memory. Today's more highly developed CD-ROM systems provide potentially limitless interactivity as their programs are typically under the supervision of an external computer.
To read CD-ROM data a laser-equipped drive in a CD-ROM player changes its rate of spin, turning the disk more slowly as sectors farther from the disk center are read. As the disk is spinning, the low-power read laser is focused on the spiral track through the thickness of the polymer disk, and reflected light is picked up by a photodetector that converts the presence or absence of indentions into electrical signals interpreted as digital data by the computer. The read laser "sees" the indentions from the side opposite where they were produced, and therefore sees an indention as a protrusion toward the laser source.
When the read laser encounters the land of an indention, the focused light from the laser is largely scattered, so the light reflected to the photodetector is diminished. In the area where there is no indention from the opposite side, more light, is reflected to the photodetector.
FIG. 2 illustrates a series of indentions formed in the surface of a CD-ROM disk 31. Surface 23 is the original disk surface, and level 25 indicates the level of penetration into the original surface of the formed indentions. The Figure is not to scale, as the indentions are almost infinitesimally small in depth relative to the thickness of the disk. The read laser operates in the direction of arrow 24.
CD-ROM, much like other high-density storage systems, relies on channel codes for storage and retrieval of data. The channel code typically used for CD-ROM is called eight-to-fourteen modulation (EFM).
In the EMF system, each time the moving disc translates an "edge" past the laser beam, the reflected light intensity changes, signalling a transition 27 that is decoded as binary 1 by the host's reading system. Binary 0's occur everywhere else, and the number of 0's between "1" transitions is a function of the length along the spiral track of the land between indentions. In operation, the channel code is converted into digital bytes and data blocks by reference to stored look-up tables. The typical system of encoding and decoding is well known in the art.
A complete CD-ROM file system consists of three major components: logical format, which defines the disk's directory structure and operating characteristics through decoder programs that determine such matters as where to find the directory of the files on the CD-ROM disk, how the directory is structured, and how to perform error correction on disk data; origination programs, which write the data on the disk according to the logical format; and destination programs, the reading component on the host computer that understands the logical format and can use it to provide access to the files on the CD-ROM and read and translate the data structure.
Most CD-ROM disks have their own operating systems that respond to calls from a file manager that is exclusively used by CD-ROM. CD-ROM disks also contain their own search decoder programs that define where sectors are located so the computer knows where to locate the stored data. These search decoders typically have a menu-driven interface on the computer. CD-ROM programs typically require 640 kilobytes of computer memory to run, and may allocate their own storage addresses on the computer's hard disk for its data access driver and destination programs.
Nearly all CD-ROM disks and CD-ROM players available today conform to what is known as the High Sierra standard or to a more recent upgrade, the ISO 9660 standard. The connection between CD-ROM players and computers also has been for the most part standardized. CD-ROM players typically use the Small Computer Systems Interface (SCSI) to link with a computer.
CD-ROMs appear to be reasonable and convenient vehicles for marketing large application programs rather than using large numbers of floppy disks. They easily provide enough space and their cost is low, about $.50 a disk when produced in large volumes. The cost of CD-ROM drives is also becoming more reasonable as well.
Despite apparent advantages, there remain some serious drawbacks to use of CD-ROMs for marketing large application packages. The most serious is that there is no reliable method or mechanism whereby comprehensive versions and features may be recorded on a single disk, and only certain portions enabled for a particular customer.
Also the lack of satisfactory copy protection of CD-ROM disks remains a problem. CD-ROM disks are typically shipped with a companion floppy disk that contains data access driver and destination programs that must be installed on the user's computer in order for CD-ROM to run. The floppy is copy-protected but the CD-ROM disk is not. This method of copy protection is no more effective than copy protection for floppies in general. The contents of CD-ROM can be copied to hard disk and the floppy protection scheme can be defeated by any of a number of available copy programs.
Multi-application packages such as Ethernet, Windows, and AutoCad, and many others, therefore, continue to be furnished on multiple floppy disks. Many of these programs have become so diverse that as many as 20 to 40 floppy disks may be required to install a complete set of programs on a host computer.
What is needed is a means whereby vendors of large and diverse applications can record all of an application on a single CD-ROM, and then selectively enable access of portions of the data at time of purchase. Also a means is needed to copy-protect CD-ROM disks so floppies are not the sole means for protection. With these improvements a vendor could cost-effectively mass-produce one full version of a product on a single CD-ROM disk, have the disk copy-protected, and selectively control use of its applications. The user, who only has to make a one-time purchase of a basic package on a single disk, could have access to other applications on the disk by simply paying the vendor for them as needed.